Longest Substring Without Repeating Characters

Given a string s, find the length of the longest substring without repeating characters.

 Example 1:

Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.

Example 2:

Input: s = "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:

Input: s = "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3.
Notice that the answer must be a substring, "pwke" is a subsequence and not a substring.

Example 4:

Input: s = ""
Output: 0

Java (Using HashMap)

class Solution {
    public int lengthOfLongestSubstring(String s) {        
        int n = s.length(), ans=0;
        Map<Character, Integer> map = new HashMap<>();
        for(int i=0, j=0; j<n; j++) {
            if(map.containsKey(s.charAt(j))) {
                i = Math.max(map.get(s.charAt(j)), i);
            }
            ans = Math.max(ans, j-i+1);
            map.put(s.charAt(j), j+1);
        }
        return ans;        
    }
}

Time complexity : $O(n)$. Index $j$ will iterate $n$ times.
Space complexity (HashMap) : $O(min(m, n))$